This invention relates to the field of spread spectrum communications and, in particular, to communications at enhanced data rates. This invention is applicable and useful for both forward as well as reverse links when the total peak instantaneous throughput from the transmitting spread spectrum system is less than the chipping rate as well as when the instantaneous throughput from the transmitting spread spectrum system is more than the chipping rate. For convenience in exposition, this disclosure will emphasize the method as applicable to the forward link.
Numerous proposals have been made to support high data rate services in various spread spectrum communication technologies. As an evolution of IS-2000-1X, a proposal commonly referred to as HDR (high data rate) has been standardized to support high data rate services within the 1.25 MHz spectrum. In addition, proposals including the 1xEV-DV proposal have been put forward to support high data rate services as well as low data rate services within the same 1.25 MHz band. The high peak data rates in the above proposals are achieved using a combination of Walsh code aggregation, Walsh variable spreading factor method, and the use of higher order constellations such as 8-PSK, 16-QAM and 64-QAM.
FIG. 1 depicts an illustrative transmitter 100 in such a system. The transmitter comprises an encoder 110, an interleaver 120, a M-ary modulator 130, a serial-to-parallel converter 140, an array 150 of multipliers, a combiner 160, a multiplier 165 and an adaptation unit 170. Encoder 110 and interleaver 120 are conventional. Modulator 130 can be any one of a variety of M-ary modulators that encode a plurality of input bits using a higher order constellation such as QPSK, 8-PSK, 16-QAM or 64-QAM. The array of multipliers 150 code the modulated data from modulator 130 and converter 140 using Walsh codes W1 through WN where N is the number of parallel streams from serial-to-parallel converter 140. Multiplier 165 spreads the data from combiner 160 using a pseudorandom noise (PN) code. Adaptation unit 170 adjusts the encoder 110, the modulator 130, and the serial-to-parallel converter 140 in accordance with the transmission characteristics of the transmission channel. In particular, adaptation unit 170 adjusts these elements so as to maximize the data transmission through the channel for the conditions then prevailing in the channel.
In operation, a data stream having a bit rate of R bits/second is received at an input to encoder 110 and is encoded as a bit stream of RS bits/second. The bit stream of RS bits/second is provided to interleaver 120 which scrambles the order of the bits so as to provide protection against transmission errors. The interleaved bit stream is then provided to M-ary modulator 130 which converts a plurality of input bits into an output symbol in accordance with the particular modulation technology used by the modulator. The serial stream of output symbols from M-ary modulator 130 is then converted to a parallel stream by serial-to-parallel converter 140 and the individual symbols in the parallel stream are then multiplied with Walsh codes W1 through WN. Finally, an output signal is formed by combining the Walsh-coded signals at combiner 160, spreading the signals by multiplying them with a pseudorandom noise signal and providing the spread signals to a transmitting antenna (not shown). Thus, a plurality of Walsh-encoded modulated symbols are transmitted simultaneously.
At the receiver, the symbols received at any time are separated by despreading the signals and multiplying them with Walsh codes and the separated signals are then demodulated in conventional fashion.
The use of higher order modulation techniques to achieve high data rates has several disadvantages. It requires a more complex and expensive RF front end arising from such requirements as the use of A/D converters having more bits, high rho-values to reduce the radio noise floor, and higher backoff for the power amplifier and results in lower ICI for the pulse shaping waveform, a lower noise figure, and greater sensitivity to inner receiver errors.
Code aggregation has certain advantages over the other two techniques depending on the operational scenario. However, if the code signals that are aggregated are only Walsh codes, the size of the set of Walsh codes limits the number of code signals that may be aggregated. In particular, the number of different Walsh codes, or of any set of orthogonal functions, of length N is only N. If any more code signals are to be aggregated, these would need to be non-orthogonal to the set of N Walsh codes.
In certain prior art operational scenarios in which the system is Walsh code-limited, alternative sets of orthogonal functions called quasi-orthogonal functions (QOF) have been included in the IS-2000 standard to accommodate more users. QOF's are functions that are generated by applying certain masks to each codevector in the Walsh code. A mask is simply a vector with 4-phase symbols (i.e., from the set {±1; ±j}) of length N. If the set of Walsh codes of length N is denoted as WN, applying a mask m to a codevector wεWN refers to the componentwise multiplication of the vectors m and w to give the new vector wm=w·m of length N, where · denotes componentwise multiplication. For further information on QOF's see U.S. Pat. No. 6,240,143 which is incorporated herein by reference.
Applying the mask m to all the codevectors in WN (denoted as WNm), one obtains N additional codevectors. Thus, using a total of M masks on WN, the size of the code signal set will be MN. Three masks in addition to the trivial mask, have been defined in IS-2000 such that the QOF's from different QOF sets have minimal cross correlation with each other as well as with the set of Walsh functions. See Tables 3.1.3.1.12-2 and 3.1.3.1.12-3 of the IS-2000 Standard.
The QOF's have minimal mini-max cross-correlation with the set of Walsh functions and therefore are optimal in this regard. Further, the QOF's have equal cross-correlation with every Walsh function of the same length. Further, the QOF's have certain optimal cross-correlation properties with Walsh functions of shorter length as would be needed with the use of shorter spreading factor Walsh functions for higher data rates.